Selectively-Lethal Weapon

Sometimes a character has one weapon, that regardless of how it used, and how improbable it would be, can be either lethal or nonlethal. They throw one, and it cuts through a steel cable, next throw, it knocks someone out without even drawing blood. This is the "selectively lethal weapon", a weapon that should be either always lethal, or always nonlethal, but somehow manages to be a bit of both, usually with no, or an extremely tenuous explanation. Contrast with Stun Guns, which are something a bit different.

Examples of Selectively-Lethal Weapon include:

Batarangs qualify, at times sharp enough to cut steel, other times they only knock you out. Occasionally they blow up in your face.
- Batman probably does have more than one type.
  - Batman: Arkham Asylum does give you multiple types of Batarang, one of which will short circuit the electronic collars the goons are wearing.
- A diagram in one book showed that the front "round part" is hard and blunt, perfect for knocking thugs out. The back part with all the spiky bits is razor sharp. Batman can throw it so that whichever end he wants strikes the target.
Sokka's Boomerang in ~Avatar: The Last Airbender~ is similar to the above Batman example. While it is used to cut and knock out, Sokka is often shown to be sharpening one edge of it while leaving the other edge dull. It can be assumed he chooses which side to throw it on.
Xena's Chakram: it often went from blunt enough to KO gangs of mooks to being able to slice through armor... occasionally in the same episode. Occasionally in the same throw.
Samurai Jack has a magical sword which will not harm those pure of heart - on the occasion where it was used against him, it merely bounced off, and when stolen and used against others, the best it can do is knock people away.
Dungeons and Dragons has merciful weapons that are enchanted to deal an extra 1d6 damage, but have all the damage nonlethal. This ability can be switched on and off as a free action, and KO'd enemies can be killed at your leisure, so there's not even a drawback. Other weapons can be used to deal nonlethal damage, but at a penalty to accuracy. And just to round out the trope, normally nonlethal weapons can take this same accuracy penalty to deal lethal damage.
- In the 4th Edition you can declare any attack to be retroactively non-lethal, up to and including a disintegrate spell.
Star Trek phasers justify this with having multiple intensities, including a stun setting. Although some have noticed a tendency, regardless of other circumstances, for the lethality of phasers to be inversely proportional to the importance of the character they're being fired at.
In Werewolf: The Apocalypse, the Children of Gaia, a tribe of pacifists, have a spell that somehow turns their teeth and claws into nonlethal damage, allowing them to take down enemies with full force but avoid killing them.
In One Piece, Zoro's sensei is able to not cut paper.
Galaxy Rangers did a nice dodge on this trope. Early on, Zachary would sometimes explicitly order his team to "set blasters for stun." Later in the series, when they were trying to score a toy deal, they had a crook say that Ranger blasters didn't have a kill setting. Now, seeing as the source of the information was a very dumb crook, the writers could both have the kid-friendly "stun only" mention on camera and a wink to the more likely prospect that there was a kill setting. Likewise, Crown blasters also had stun and kill settings, but it was justified in that the Queen wanted humans (or other compatable species) as fodder for the Psychocrypt, and you couldn't drain Life Energy from dead enemies.
The titular sword of the Sword of Truth works this way - the sword can only cut if the wielder believes the target is an enemy except in its super-mode that he unlocks near the end of the first book. The wizard who gave Richard the sword stresses that the user doesn't have to be right in their beliefs, they merely have to believe. Which is why wizards agonize over who gets the title that the sword goes with.
The Medic Crossbow in Team Fortress 2 heals allies and damages enemies. Which makes it hilarious to fire into the melee on the 3rd point in Medieval Mode.